The present invention relates to a deflection compensating roll for arrangement parallel to a counter roll, and more particularly to a deflection compensating impression roll disposed parallel to a gravure cylinder in a gravure press.
In many printing, coating or laminating operations, where it is desired to pass a web or several webs between two rollers, it is essential that the pressure exerted by the rollers against the web is uniform across the width of the web. Similarly, when printing ink is distributed by passing between a roller having a metal surface and a roller having an elastomeric covering, it is advantageous that the contact pressure between the rollers be uniform across the width of the rollers. Even if the rollers are of proper cylindrical shape, and their bearings are properly aligned, uneven contact pressure can result from the bending deflection of one or both of the rollers due to the contact pressure.
In rotogravure printing, small cells representing the image to be printed are etched or engraved in the surface of the gravure cylinder. In those areas of the gravure cylinder where print-out is required, there may be approximately 10,000 to 40,000 cells per square inch. In those areas where a dark tone is to be printed, the cells are deeper and/or of greater surface area then in those areas where a light tone is to be printed.
In a conventional gravure press, the gravure cylinder is rotated around its horizontal axis with its lower surface immersed in a fountain containing liquid ink. Rotation of the cylinder carries the ink flooded portion of the cylinder out of the fountain and passes it under a doctor blade whose edge engages the surface of the cylinder and removes the ink that is clinging to the surface of the gravure cylinder, leaving only the ink that is located in the cells.
The print-out or transfer of the ink that remains in the cells to a printing substrate, which may be a web of paper, paper board, glassine, metal foil, film, or a laminate of the above materials, is accomplished by pressing the substrate web into contact with the inked and doctored portion of the rotating gravure cylinder by means of an elastomeric covered impression roll which rotates around a horizontal axis arranged parallel to the axis of the gravure cylinder.
The impression roll includes a tubular steel impression roll core covered with an elastomeric covering. The elastomeric covering is generally made from such materials as natural or synthetic rubbers filled with carbon black or zinc oxide, polyurethane, or similar materials. The elastomeric coverings are typically from 0.375 to 0.750 inches thick and have a hardness of 75 to 95 Shore A Durometer. Softer coverings are generally used on smooth foil and film, where low impression pressures are employed.
In order to obtain the optimum print-out across the width of the web and to avoid tears and wrinkles in the web, it is essential that the impression pressure is uniform across the width of the impression roll covering. The deleterious effects of uneven impression pressure are most pronounced when the distance between the center line of the gravure cylinder and the center line of the impression roll differs by more than about 0.003 to about 0.007 inches across the width of the impression roll covering.
The forces that are applied to the impression roll to press the substrate against the gravure cylinder and thus cause the ink to transfer to the substrate are adjusted in accordance with the hardness and roughness of the side of the printing substrate that is printed, i.e., harder and rougher substrates require higher impression pressures. Paper, such as that used in magazines and catalogs is typically printed at impression pressures of about 40 to about 80 pli (pounds per linear inch of impression roll covering face width).
For gravure presses which print webs up to about 50 inches wide, impression rolls which have outside diameters of up to about 9 inches are sufficiently stiff so that the effects of uneven impression pressure due to bending of the impression roll core are minor. On presses which use wider webs, bending of the impression roll can cause poor printout near the center of the web because of insufficient impression pressure, as well as damage to the impression roll covering and wrinkles and tears in the web.
For wide presses, the conventional practice has been to place a heavy steel back-up cylinder, e.g., 12 inches in diameter, on top of and in pressure contact with the impression roll. The impression pressure is developed by the dead weight of the back-up cylinder, and the application of forces at the bearing blocks of the back-up cylinder near the side frames of the press. Such an arrangement greatly reduces the bending of the impression roll and is effective up to a point where the maximum web width used with the gravure press is not more than 6 to 7 times the diameter of the gravure cylinder. If the web width for which the gravure press is designed is larger than 6 or 7 times the diameter of the gravure cylinder of if a gravure cylinder of small diameter is used, the deleterious effects of uneven impression pressure due to bending of the gravure cylinder is noticeable.
However, the use of a back-up cylinder also has certain drawbacks. The impression roll covering is compressed twice during each rotation of the impression roll. This increases the press power requirements and causes increased heating of the impression roll covering, thereby shortening its life. Further, the added rotary inertia of the back-up cylinder strains the drive components of the press during acceleration and emergency stops.
In gravure presses, gravure cylinders and impression roll cores are presently proportioned so that an increase in wall thickness will not significantly increase the resistance to elastic bending. Moreover, their diameters cannot be arbitrarily increased because the gravure cylinder circumference must be a simple multiple of the page width or length or the repeat length of the pattern that is printed. Further, with an impression roll having a substantially larger than customary diameter, the impression forces are distributed over too wide an impression flat width thereby reducing the pressure per unit of area in the contact zone between gravure cylinder and impression roll, thus impairing print-out.
In response to the aformentioned problems a number of deflection compensating impression systems that operate without back-up cylinders have been introduced for gravure presses. The NIPCO roll, manufactured by Escher Wyss Ltd. of Zurich, Switzerland, employs a non-rotating beam across the width of the press into which a row of hydraulic cylinders have been incorporated. Associated downward pointing pistons bear against a rotating steel reinforced rubber sleeve, which exerts impression pressure on the web. Controlled leakage of the hydraulic fluid provides lubrication between the stationary pistons and the rotating sleeve, and also provides cooling. Pressure is applied to only that portion of the impression roll in contact with the web.
Other deflection compensating impression systems attempt to apply essentially uniform impression pressure across the entire width of the impression roll face. Such systems are the Bugel roll manufactured by M.A.N. of Augsburg, West Germany; the CDR Controlled Deflection Roll manufactured by the Motter Press Company of York, Pa.; the Flexible Impression Roll manufactured by Componenti Grafici of Lomellina, Italy; and the K2 Roller System manufactured by Albert-Frankenthal AG in Frankenthal, West Germany. All of these systems employ a stationary inner beam and a tubular elastomeric covered rotating metal shell that is supported by ball or roller bearings near its ends. To overcome the effects of impression roll and gravure cylinder bending, downward forces are applied to the inner rings of ball or roller bearings, whose outer races bear against the inner surface of the tubular impression roll core near the center of the impression roll. Except for the CDR roll, the pressure on the bearings near the center of the impression roll is applied by pneumatic or hydraulic means.
With the above systems, the pressure that is applied at the center bearings has to be released by separate, external, manual or automatic means to permit free rotation of the impression roll when the impression roll is lifted off the gravure cylinder for insertion of a new web which occurs at the beginning of the press run or after a web break. Moreover, the pressures that are applied near the roll centers must be readjusted every time the pressures applied to the ends of the impression roll are changed.
Systems have been proposed that eliminate the need for center pressure adjustments and that enable the impression roll shell to turn freely when the impression roll is lifted off of the gravure cylinder to pass a web therebetween at the beginning of the press run or after a web break. In such systems, the roller shell is supported on a stationary beam by two bearings that are located a given distance away from the roller ends towards the center of the press. By methods outlined in the literature, e.g., "Formulas for Stress and Strain", Fifth Edition, by R. J. Roark and W. C. Young, McGraw-Hill, Inc. 1975, International Standard Book Number 0-07-053031-9, it can be demonstrated that the upward deflections at the impression roll center and at its ends are equal under load when the bearings are located at a distance of about 22 percent of the roller face length as measured from the ends of the roller. However, such systems are not satisfactory when the gravure cylinder deflects by more than about 0.003 inches or when gravure cylinders having different diameters and bending stiffness are used on a gravure press.